Certain research and manufacturing processes require the use of a process chamber with high vacuum. For example, in semiconductor wafer processing, vacuum is used during many thin-film deposition and etching operations, primarily to reduce contamination. In such processes, pumps capable of producing a “high vacuum” of 10−6 Torr or lower are useful to assure adequate pumping speed at process pressure, and to allow for a low base pressure for cleanup between steps.
Several currently-available vacuum pump configurations are capable of producing and maintaining a high vacuum. One design, the turbo-molecular vacuum pump, is frequently used in both manufacturing processes and in research instrumentation. A conventional stage arrangement of a turbo-molecular vacuum pump includes a stack of alternate rotors and stators. Each stage effectively comprises a solid disc with a plurality of blades depending (nominally) radially inwardly or outwardly therefrom. The blades are evenly spaced around the circumference of the disc and angled “about” radial lines out of the plane of the disc in the direction of rotation of the rotor stage.
The turbo-molecular vacuum pump is inefficient or inoperable outside the molecular flow realm. For that reason, a commercially available vacuum pump may contain, in addition to several turbo-molecular stages, one or more molecular drag stages and one or more regenerative stages placed between the turbo-molecular stages and the pump outlet.
Turbopump rotors are frequently designed with partial or full magnetic levitation bearings. In certain instances, ceramic or other contact bearings are provided on the fore-vacuum side, and radially stabilizing permanent magnetic bearings are provided on the high-vacuum side. In certain high-vacuum applications, full magnetic levitation bearings are used to suspend the rotor. In those cases, the radial position may be regulated via a permanent magnet stabilizer, or may be regulated electronically. Electromagnets are also used to maintain the axial position of the rotor.
The level of vibration of the rotor in such a case is very low because there is no direct contact with the casing. Further, the rotor may be automatically compensated for out-of-balance vibration, reducing vibration of such rotors by a factor of 10 as compared with a similar rotor supported by ball bearings.
Other advantages of magnetic levitation bearings as compared with mechanical bearings are the absence of oil on the fore-vacuum side, and the lack of wear and resulting maintenance. For all the above reasons, magnetic levitation bearings are the suspension of choice in turbomolecular pumps designed for high and ultra-high vacuum applications.
Strong circumstantial evidence suggests that magnetically levitated rotors in turbomolecular pumps can acquire very high potentials through charge accumulation. The origin of the charge is most likely negatively charged process plasma dust. This dust exchanges its charge upon collision with the rotor, where every collision contributes an incremental amount of charge to the rotor. The rotor is thus charged to high electrical potentials. The electric field associated with the electrically charged rotor exerts a repelling force on dust particles which are caught in the non-uniform electric field at the inlet of the turbo pump to form a particle cloud. These particles may create device defects and decrease the process yield.
An additional contribution to the charge on the rotor may come from tribocharging. Tribocharging is the result of a charge exchange process when dissimilar materials come in contact with each other. For example, a person accumulates charges on the body by walking across a carpet. When contact is made with a grounded item, a discharge occurs, giving the person a shock. In the case of a Mars lander, particles in a dust storm have been known to cause an exchange of electrons, resulting in charge buildup. The amount of charge accumulation depends on the nature of the two materials that come into contact with each other and their ability to dissipate charge.
While a charged conductor distributes charges throughout the body, a charged dielectric maintains local charge distributions. In each case, the primary condition for accumulating charge is that one material is insulated from another, thus preventing recombination of the charges.
As discussed above, the rotor of a turbo-molecular pump is often magnetically levitated, and thus electrically isolated from the surrounding, grounded stator. Even pumps without magnetic levitation often have rotors suspended by ceramic bearings, also insulating the rotor from ground. Given a flow of non-ionized gas over the rotor, a static charge might accumulate on the rotor of the turbo-molecular pump during operation due to tribocharging. The materials that are present (gas species and turbo-molecular pump rotor material) and the absence of a conducting path between the charges will determine the amount of accumulated charge.
Additionally, pumping of ionized gases and dusty plasma also results in charge accumulation. Ions and charged particles contribute to the charge accumulation on the rotor by transferring their charge upon collision. The net charge on the rotor is the result of all these processes.
The build-up of a sufficiently high net charge on the rotor will result in a sudden discharge. The voltage at which discharge occurs depends on the amount of accumulated charge, the pressure, and the distance between the two oppositely charged surfaces.
Plasma physics theory explains the discharge of accumulated charge as the onset of self-ionizing electron flow triggered by an ionizing event. Self-ionizing electron flow is triggered when one electron flows, initiating the release of further electrons through impact with another atom, creating a cascade of electron release. The resultant positive ions then reinitiate further electron flow when they impact the electrodes holding accumulated charge. The onset of self-ionizing electron flow depends on the gas species, the materials that are holding the charge, the pressure, and the distance between the two materials.
Paschen studied the phenomenon of breakdown voltage through a series of experiments in which a pair of electrodes were placed in a vacuum. A voltage was applied to the electrodes. The voltage at the point of breakdown was measured. Paschen found that the breakdown voltage depends on the gas species, the distance between the electrodes, the material of the electrode, the shape of the electrodes and the pressure of the gas. A series of curves were thereby developed for a combination of materials and conditions. Townsend's equations provide for a numerical calculation of the breakdown voltage once certain parameters of the gas species and electrode materials are known.
A typical Paschen curve 110 for tungsten conductors in argon gas is shown in the plot 100 of FIG. 1. A breakdown voltage 111 is plotted against a product of pressure and gap width 112. Independent of conductor material or gas species, Paschen curves have a similar shape in which the breakdown voltage will be minimum at some intermediate pressure-distance product. From the graph of FIG. 1, for a 0.3 mm gap the minimum voltage will be approximately 115 V at a pressure of 16 Torr. For nitrogen, the minimum voltage would be closer to 250 V.
All Paschen curves have a minimum breakdown voltage for a certain pressure-distance combination. Considering a fixed gap, pressures below the minimum voltage point will result in an exponentially rising breakdown voltage approaching infinity. Thus, the system exhibits signs of a perfect vacuum. Pressures greater than the minimum voltage point result in ever increasing breakdown voltages. The converse is also true. However, in interpretation of a fixed pressure with variable distance, the graph should be interpreted only in the case where the distance is large and decreasing.
Turbomolecular pump rotors are operated under conditions wherein a gap or insulating ceramic bearings are present between the rotor and ground, electrically isolating the rotor. Relative movement between the rotor and particles in the pump results in charging which, as noted above, can contribute to several problems including trapping particle dust. There is therefore presently a need to provide an improved turbo-molecular pump incorporating a solution to the problem of accumulating charges on the rotor during operation. To the inventor's knowledge, no such pump is presently available.